1. Field of the Invention
The invention relates to a semiconductor device and a method of fabricating the same.
2. Description of the Related Art
In these days, a semiconductor memory device has been required to operate at a higher rate, have a greater capacity for storing data therein, and operate in less power consumption. To such requirements, there has been suggested a dynamic random access memory (DRAM), which is much in demand now, having a capacitive or insulating film of a capacity device or a memory node which film is composed of metal oxides having a greater dielectric constant than that of a silicon dioxide film in order to make it possible to form memory cells in a circuit smaller. Some of such metal oxides are ferroelectric. There has been also suggested a non-volatile memory utilizing ferroelectric characteristics of those metal oxides.
Japanese Unexamined Patent Publication No. 7-38068 published on Feb. 7, 1995 has suggested a semiconductor memory device having a capacitive film composed of high-dielectric substance. FIG. 1 illustrates the suggested semiconductor memory device.
LOCOS oxide films 803 are formed at a surface of a silicon substrate 801 to thereby define a device formation region therein. Below LOCOS films 803 are formed channel stopper regions 804. Source/drain regions 802 are formed at a surface of the silicon substrate 801. Gate electrodes 805 acting as word lines are formed on LOCOS oxide films. A signal line 806 acting as a bit line is formed on the silicon substrate 801 between the source/drain regions 802.
An interlayer insulating film consisting of a silicon dioxide film 807 and a silicon nitride film 808 is formed on the silicon substrate 801. The interlayer insulating film is formed therethrough with contact holes 809 reaching the source/drain regions 802. Each of the contact holes 809 is filled with a plug 810 composed of an electrically conductive material such as metal.
A capacity device having a capacitive film composed of high-dielectric substance is formed on the interlayer insulating film. The capacity device is in electrical connection with the source/drain regions 802 through the plug 810. The silicon nitride film 808 and the plug 810 are both planarized at the same level.
The capacity device is comprised of a plurality of lower or storage electrodes 811, a capacitive insulating film 812 entirely covering the lower electrodes 811 therewith, and an upper or plate electrode 813 deposited all over the capacitive insulating film 812.
Over the upper electrode 813 are formed a metal wiring layer (not illustrated), and an interlayer insulating film 814 for electrically insulating the metal wiring layer and the upper electrode 813 with each other, to thereby constitute a semiconductor memory device.
After the metal wiring layer has been formed, the product is thermally annealed in hydrogen atmosphere in order to reduce a dispersion in a threshold voltage of transistors arranged on a surface of the silicon substrate, and a dispersion in a current for driving transistors. This thermal annealing compensates for defects, such as a trap level, formed at an interface between the capacity device and the interlayer insulating film making contact with the capacity device.
However, the above-mentioned semiconductor memory device illustrated in FIG. 1 has shortcomings as follows.
First, since the thermal annealing in hydrogen atmosphere is carried out after the capacity device has been formed, the capacitive film composed of metal oxide is unpreferably reduced with the result of deterioration of capacitive characteristics of the capacity device.
Secondly, transistors formed on layers located below a layer on which the capacity device is formed may be deteriorated with respect to performance and reliability thereof. Specifically, as illustrated in FIG. 1, since the silicon nitride film 808 entirely covers the silicon dioxide film 807 except the plugs 810, if the product is thermally annealed in hydrogen atmosphere after the capacity device has been formed, the silicon nitride film 808 acts as a barrier for hydrogen to reach layers on which transistors are formed. As a result, performance of transistors and reliability for transistors are deteriorated, and a characteristic of transistors is not uniformized.
In view of the above-mentioned problems in a conventional semiconductor memory device, it is an object of the present invention to provide a semiconductor device and a method of fabricating the same both of which are capable of avoiding a capacitive film composed of metal oxide from being degraded even if a capacity device is thermally annealed in hydrogen atmosphere.
In one aspect of the present invention, there is provided a semiconductor device including (a) a semiconductor substrate, (b) a capacity device, (c) an interlayer insulating layer formed between the semiconductor substrate and the capacity device for electrically isolating them with each other, the interlayer insulating layer being formed therethrough with a contact hole below the capacity device, (d) a contact plug composed of an electrically conductive material and formed in the contact hole, (e) a film composed of a material through which hydrogen is not allowed to pass, the film entirely covering both the capacity device and the contact plug therewith.
In accordance with the above-mentioned semiconductor device, both the capacity device which may include a capacitive film composed of metal oxide, and the contact plug electrically connecting the capacity device to the semiconductor substrate are entirely covered with a film composed of a material through which hydrogen is not allowed to pass. Hence, it is possible to prohibit hydrogen to reach the capacitive film.
There is further provided a semiconductor device including (a) a semiconductor substrate, (b) a capacity device, (c) an interlayer insulating layer formed between the semiconductor substrate and the capacity device for electrically isolating them with each other, the interlayer insulating layer being formed therethrough with a contact hole below the capacity device, (d) a contact plug composed of an electrically conductive material and formed in the contact hole, (e) a first film composed of a first material through which hydrogen is not allowed to pass, and formed between the interlayer insulating layer and the capacity device, (f) a second film composed of a second material through which hydrogen is not allowed to pass, and formed on an inner wall of the contact hole, (g) a third film composed of a third material through which hydrogen is not allowed to pass, and formed to cover an upper surface of the capacity device therewith, and (h) a fourth film composed of a fourth material through which hydrogen is not allowed to pass, and formed to cover a side surface of the capacity device therewith.
It is preferable that the first, second, third, and fourth materials are the same. The first, second, third, and fourth materials are preferably nitride, and more preferably silicon nitride.
It is preferable that the capacity device includes a capacitive film composed of tantalum oxide. As an alternative, the capacity device may include a capacitive film composed of high-dielectric or ferroelectric substance.
It is preferable that the contact hole electrically connects the capacity device to a source or drain region formed in the semiconductor substrate.
The semiconductor device may further include an upper electrode to be electrically connected to an external wiring, in which case, the upper electrode is located remote from the capacity device.
In another aspect of the present invention, there is provided a method of fabricating a semiconductor device, including the steps of (a) forming a multi-layered interlayer insulating film on a semiconductor substrate, the multi-layered interlayer insulating film including a first film, as an uppermost film, composed of a first material through which hydrogen is not allowed to pass, (b) forming a contact hole through the multi-layered interlayer insulating film to the semiconductor substrate, (c) forming a second film on an inner wall of the contact hole, the second film being composed of a second material through which hydrogen is not allowed to pass, (d) forming a plug layer in the contact hole, the plug layer being composed of an electrically conductive material, (e) forming a capacity device over the contact hole in such a manner that the capacity device is more extensive than a cross-section of the contact hole, (f) forming a third film on the capacity device in such a manner that an end surface of the third film, a side surface of the capacity device, and the first film are exposed, the third film being composed of a third material through which hydrogen is not allowed to pass, and (g) forming a fourth film covering both the end surface of the third film and the side surface of the capacity device therewith, the fourth film being composed of a fourth material through which hydrogen is not allowed to pass.
In accordance with the above-mentioned method, it is possible to cover a capacity device with the first to fourth films in the same number of photolithography steps as the number of photolithography steps in a conventional method.
For instance, the step (c) may include (c-1) forming the second film over the semiconductor substrate so that an inner wall of the contact hole is covered with the second film, and (c-2) etching the second film back so that only a portion of the second film deposited on the semiconductor substrate is removed.
The step (e) may include (e-1) forming a lower electrode on the semiconductor substrate over the plug layer in such a manner that the lower electrode is more extensive than a cross-section of the contact hole, (e-2) forming a capacitive film entirely covering the lower electrode therewith and further covering the first film therewith, and (e-3) forming an upper electrode over the capacitive film, the third film being formed on the capacitive film.
The step (g) may include (g-1) depositing the fourth film over the capacity device and the first film, and (g-2) etching the fourth and first films so that an upper surface and the end surface of the third film, and the side surface of the capacity device are covered with the fourth film.
There is further provided a method of fabricating a semiconductor device, including the steps of (a) forming a multi-layered interlayer insulating film on a semiconductor substrate, the multi-layered interlayer insulating film including a first film, as an uppermost film, composed of a first material through which hydrogen is not allowed to pass, (b) forming a plurality of contact holes in a row through the multi-layered interlayer insulating film to the semiconductor substrate, (c) forming a second film on an inner wall of the contact hole, the second film being composed of a second material through which hydrogen is not allowed to pass, (d) forming a plug layer in the contact hole, the plug layer being composed of an electrically conductive material, (e) forming a lower electrode on the semiconductor substrate over the plug layer in such a manner that the lower electrode is more extensive than a cross-section of the contact hole, (f) forming a capacitive film entirely covering the lower electrode therewith and further covering the first film therewith, (g) forming an upper electrode over the capacitive film, (h) forming a third film on the upper electrode, (i) etching the third film in such a manner that an end surface of the third film, a side surface of the upper electrode, a side surface of the capacitive film, and the first film are exposed, and that the upper electrode is co-owned by a plurality of the lower electrodes, the third film being composed of a third material through which hydrogen is not allowed to pass, and (j) forming a fourth film covering both the end surface of the third film and the side surface of the capacity device therewith, the fourth film being composed of a fourth material through which hydrogen is not allowed to pass.
In still another aspect of the present invention, there is provided a semiconductor memory device comprising a plurality of semiconductor devices each including a transistor and a capacity device, each of the semiconductor devices being fabricated in accordance with the above-mentioned method.
It is preferable that the semiconductor devices are arranged in a grid.
It is also preferable that a contact hole for electrically connecting the semiconductor memory device to an external wiring layer is formed outside the capacity device or a region where the semiconductor devices are arranged. In accordance with this embodiment, it is no longer necessary to form a contact hole, which electrically connects a capacity device or an upper electrode of a capacity device connected to transistors to an external wiring layer, in the vicinity of the capacity device. That is, if a contact hole connecting an upper electrode to an external wiring layer were formed in the vicinity of a capacity device, hydrogen would readily diffuse into the capacity device through the contact hole during thermal annealing in hydrogen atmosphere. To the contrary, by positioning a contact hole remote from a capacity device, it would be possible to avoid degradation of a capacity device.
The above and other objects and advantageous features of the present invention will be made apparent from the following description made with reference to the accompanying drawings, in which like reference characters designate the same or similar parts throughout the drawings.